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#15-13 Geotechnical testing and investigations

Published: | Category: Technical advice note | Audiences: Road controlling authorities, Road traffic engineers & consultants, Roading contractors

The NZ Transport Agency is currently participating in the preparation of an industry ground investigation specification which may be adopted in the future for NZ Transport Agency projects.

Publication details

The NZ Transport Agency is currently participating in the preparation of an industry ground investigation specification which may be adopted in the future for NZ Transport Agency projects. 

In the interim, the Agency is aware of a number of practices that may be affecting the quality of testing data and therefore geotechnical design(s) and thereby structure designs. This TAN relates to those practices and presents the Transport Agency’s interim requirements pending the new ground investigation specification.

Standard penetration testing (SPT)

It is noted that variation in equipment being used to conduct SPT tests is resulting in varying hammer efficiencies. Some ground investigation suppliers do not appear to either determine hammer efficiency or report it. Such variation can have a considerable effect on calculated N60 values. SPT hammer efficiency testing must be undertaken and conducted in accordance with ASTM D4633-10(external link) and reported with the associated SPT test results. 

An increasing practice of undertaking SPTs in boreholes affected by base heave without any measures to mitigate or control it (for example water, drilling fluids or similar within the hole to counter effects) has been observed. It is expected that drilling contractors will have, within their QA and QC procedures, appropriate identification of, and mitigation measures for, such situations that shall be adopted by site crews during the physical works.

NZ Transport Agency consultants/suppliers procuring and/or supervising ground investigations on behalf of the Transport Agency shall ensure hammer efficiency testing is being undertaken and reported. Hammer efficiencies shall be reported within factual reports, clearly identifying which tests were conducted by which hammer. Similarly, confirmation of drilling procedure to mitigate base heave shall be confirmed.

Cone penetrometer testing (CPT)

It has become apparent that some CPT operators do not adequately check drift or calibration prior to commencement of testing.

NZ Transport Agency consultants/suppliers procuring and/or supervising ground investigations on behalf of the Transport Agency shall ensure compliance and check that cones are appropriately calibrated and in acceptable condition. Calibration certificates for cones used shall be included in factual reports.

Logging of exploratory holes

Exploratory holes includes all ground penetrations and excavations undertaken to assess ground conditions, including, but not limited to, hand augers, test pits, boreholes, window sampling and pavements pits.

The following requirements shall be followed unless expressly agreed otherwise with the Transport
Agency:

  • All logging is to be undertaken in accordance with the current version of the New Zealand Geotechnical Society Guidelines Field description of soil and rock(external link) by suitably qualified person;
  • For machine drilled boreholes, the method of drilling shall be clearly noted including sonic drilling, which shall be annotated as such;
  • Hand shear vanes shall be conducted in accordance with the current version of the New Zealand Geotechnical Society Guidelines for hand shear vane testing(external link);
  • With the exception of pavement pits, all logs shall be provided electronically (separately from any required factual reporting) in AGS4(NZ) format and ‘up-load’ ready for the NZ National Geotechnical Database. To avoid any potential ‘commercial-in-confidence’ infringement, these shall be provided (separately from any reporting) to the Transport Agency’s Principal Geotechnical Engineer (through the Transport Agency’s Project Manager) who will arrange uploading.

Core imagery

Core images (photographs) have been noted to be of declining clarity and therefore value due to lack of compliance with the requirements of the NZ Transport Agency’s Contract for Professional Services contained within SM030 (State highway professional services contract proforma manual).

The Transport Agency reminds all parties that there are specific requirements within SM030. Core images should be taken before cutting/breaking/handling that forms any part of the logging process. Clearly where core deconstruction reveals aspects of interest, further imagery would be valuable and anticipated.

NZ Transport Agency consultants/suppliers procuring and/or supervising ground investigations on behalf of the Transport Agency are reminded, and shall ensure that images are clear and satisfy the recommendations of SM030.

Investigation testing procedures

The Transport Agency reminds all parties that where specific New Zealand and other standards are required to be adopted, it is essential that such standards are followed unless specifically agreed with the Agency and clearly defined within the factual reporting. Subsequent users of factual information rely upon testing being conducted in accordance with such standards.

Application

Given that the above items are essentially existing requirements which should currently be being followed, no specific date of application applies. However, the noted reporting requirement (for SPT and CPT testing) shall take effect from 1 January 2016. As these relate to QA practices and procedures no additional costs to the Transport Agency are anticipated for such reporting.

Compliance checking

From 1 January 2016 those parties engaged to supervise and to review ground investigation reports for and on behalf of the Transport Agency shall ensure appropriate reporting is undertaken.

Further information

Contact Stuart Finlan of the Highways and Network Operations’ Structures team at stuart.finlan@nzta.govt.nz.

Kevin Reid

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